1. Field of the Invention
The present invention relates to an image formation device which can carry out the normal development and the inverted development by switching between them, and to a microfilm reader-printer which includes such an image formation device.
2. Description of the Prior Art
In a prior-art microfilm reader-printer of the above kind, if the paper to be used for copying is set to be of the A4 size, then there is formed an image of A4 size on the photosensitive drum. In transferring an image formed on the photosensitive drum to a copying paper, the leading edge of the image on the photosensitive drum and the leading edge of the paper are set to be flush with each other.
Now, if a letter paper is used as the copying paper in the above, its length is not large enough to cover the end portion of the image on the drum. As a result, development and transfer will continue to be carried out even after the end of the paper passed the transfer position, and the toner on the photosensitive drum makes contact with the transfer charger, which may pollute the toner.
In so doing, the toner will be wasted. In addition, due to the toner pollution by the transfer charger, especially in a reader-printer designed for image pickup from microfilms, there was a problem that the transfer cannot be carried out properly since the microfilm has a black background in the margins. Further, in an image formation device which carries out the inverted development, an exposed portion of the photosensitive drum is developed and there exists, in general, a rise or a fall time of the exposure lamp when it is turned on or off, that is, it takes 1 or 2 seconds before the exposure lamp rises or falls completely.
For this reason, the portions that come before the leading edge and after the trailing edge of the effective image formation region are also exposed and developed. This results in a problem which brings about wastes of the toner and an increase in the burden to the cleaner of the residual toner.
Moreover, in an image formation device which can be switched between the normal development and the inverted development, in the case of carrying out an inverted development, the photosensitive drum will be developed if it is not given a sufficiently high potential. Therefore, development of the nonimaging section in the axial direction of the photosensitive drum is prevented by setting the charging width to be greater than the exposure width and the exposure width to be slightly wider than the paper width.
Further, as for the developing width, it used to be set somewhat wider than the paper width, since the development at the end portions of the paper will become unsatisfactory if the developing width approaches the paper width. Accordingly, the relationship between the various widths is given by "charging width &gt; developing width &gt; exposure width &gt; paper width." However, when normal development is carried out using such an image formation device, the nonimaging portion is also developed so that it leads to a waste of the toner and to an increase in the burden to the cleaner which cleans the residual toner.
On the contrary, in an image formation device which is set for the normal development as "exposure width &gt; developing width &gt; charging width &gt; paper width," areas outside of the charging region of the photosensitive drum will be developed in the inverted development, so that there is also a problem of bringing about a waste of the toner. In other words, in the existing image formation device, there was a problem that the toner is wasted when switching between the inverted development and the normal development is carried out.
Further, in an image formation device such as a microfilm reader-printer, even when the image formation operation is started under a condition in which the developing apparatus is not set, the image will never be copied on the paper. Because of this, if the system is designed such that all of the functions are to be interrupted when both of the developing apparatuses are removed, the system cannot be used as a microfilm reader, in spite of the fact that it should still be usable in principle for that purpose.